Leaf area index (LAI) and net photosynthetic rate (Pn) of maize were measured with an infrared CO2 analyzer in the lax type corn hybrid “Jidan 159", the mid type “Nongda 3138" and the compact type “Simi 25" to study the relationship between LAI and Pn. The results showed that the variations of LAI and Pn of the three varieties were a single peak curve through the whole growth stage at the optimum density with the maximum value at the filling stage. LAI and Pn of “Simi 25" are higher than that of other varieties during the growth stage. Pn of decreases with the increase of LAI. The decrease of Pn was less in “Simi 25" compared with that in “Jidan 159" and “Nongda 3138". Pn of the upper leaves were higher than that of middle leaves and the latter was higher than that of lower leaves. It could be concluded that the suitable LAI and Pn for high yield of maize was 4.5～4.6 and 36.0～39.0 μmol/(m2*s), respectively.%为探明玉米叶面积指数(LAI)与光合速率(Pn)变化关系的规律，用远红外线CO2分析仪,对平展型（吉单159）、中间型（农大3138）、耐密型（四密25） 3种株型玉米品种在适宜密度下群体冠层Pn和LAI进行了测定。结果表明： 3种株型品种在其最适种植密度下LAI和Pn在整个生育期内均呈单峰曲线变化，于灌浆期达最大值，且“四密25”的LAI和Pn大于其它2个品种。随种植密度增加，群体LAI增加而单叶Pn降低，其中“四密25”降幅最小。冠层内单叶光合速率的变化规律是上层叶>中层叶>下层叶。高产群体的适宜LAI为4.5～4.6,单叶平均Pn(CO2)为36.0～39.0μmol/(m2*s)。
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